Hi , I have a harbor breeze fan and need to replace the old light pull chain. We picked up a kit , but this one has three wires whereas the exsisting pull kit only has two. What wire can we not use? The colors are red, blue and black?

I think you need to return that switch kit and get the correct two wire switch. It is likely packaged almost identically hanging right next to the one you grabbed. They are not interchangeable.

If you have 4 bulbs, you could hook up 2 opposite lights on on one wire, the other 2 lights on another wire, and the third wire goes to hot(maybe from wall switch).
You need to determine the common which goes to hot. Meter or continuity tester would tell.
Miswiring will do nothing but not function correctly. You cannot harm by miss wiring the 3 connections.